1. Field of the Invention
This invention relates to the manufacture of tinted hydrogel materials. More specifically, this invention relates to the manufacture of tinted hydrogel materials, such as contact lenses, wherein the tint is achieved by use of vat dyes.
2. State of the Art
For many years, soft hydrophilic contact lenses have been produced as clear, colorless products. Although clear contact lenses ensure that the lenses are hardly noticeable on a patient's eye, these lenses are difficult to locate when lost or dropped on a randomly colored surface. In order to alleviate this problem, a tint is conventionally added to the lens. This tint may be throughout the whole of the lens, over the whole of an area banded by the iris diameter, leaving the perimeter clear, or even only that area in a doughnut shape which would be superadjacent to the iris. The tint may be deep in color and some lenses have been developed to enhance the color of the iris. Other tints are sufficiently light so as not be noticeable on the eye or interfere with color perception, yet be intense enough to allow the lens to be easily located. This latter type of tint has often been described as a handling tint, "locator" tint or "visibility" tint.
The application of a handling tint has usually been made to a lens after the lens has been fully manufactured to its final hydrated form. Such methods are described in U.S. Pat. Nos. 4,891,046 and 4,157,892. U.S. Pat. No. 4,891,046 describes tinting a hydrophilic contact lens with a dichlorotriazine dye in a two step procedure. In the first step of the procedure, the lens, which is formed from a hydrophilic polymer obtained by the peroxide-initiated polymerization of a polymer forming composition, is immersed in an aqueous solution of dichlorotriazine dye. Under specific conditions, the dye diffuses into the lens. Thereafter, the dye-impregnated lens is immersed in an aqueous solution of base which catalyzes the reaction of the dye with the hydroxyl groups on the polymer backbone which results in covalent incorporation of the dye onto/into the polymer backbone. Accordingly, such processes involve covalent incorporation of the dye to the polymer backbone in a procedure separate from the preparation of the lens (polymer) itself which disadvantageously extends the manufacturing process and increases costs.
Covalent incorporation of such reactive dyes at a monomeric stage has also been disclosed in the art. For example, U.S. Pat. No. 4,997,897 describes a method of incorporating monomerically reactive dyes at the monomer stage so that the dye becomes part of the lens polymer matrix. The monomeric reactive dyes described in this reference require a multi-step synthesis so as to incorporate the reactive vinyl functionality onto the dye molecule. In this way the dye becomes covalently integrated into the polymer during polymerization. However, such multi-step syntheses to form monomers useful in hydrogel formation disadvantageously extend the manufacturing process to include synthetic steps to the dye prior to polymerization.
Similarly, U.S. Pat. Nos. 5,151,106 and 5,292,350 describe methods for tinting a hydrophilic polymer by incorporating a reactive dye into monomers used during polymerization. The reactive dye is physically entrained within the polymer composition and, subsequently, the polymer composition is contacted with an aqueous solution of a base which catalyzes the covalent attachment of the reactive dye to the polymer. For example, the --OH group of the hydroxyethyl methacrylate polymer, under alkaline conditions, will form an ether bond with the reactive dye. This method requires the reactive dyes to be soluble in the monomer mix and requires exposure to a "fixative" environment during or after the lens hydration. According to U.S. Pat. No. 5,151,106, oxidizing initiators cannot be used for conducting the polymerization of the lens polymer in this method because many of the reactive dyes are sensitive to discoloration by oxidation. A further disadvantage to this method is that extra steps are required in the manufacturing process to rinse and reequilibrate the lens for storage in physiological saline.
Accordingly, a common aspect associated with covalent attachment of a dye to the polymer backbone either during or after polymerization is that such attachment requires additional process steps so as either to covalently attach the dye to the formed polymer or to modify the dye to include a reactive vinyl functionality which will permit incorporation of the dye into the polymer during polymerization. However, such additional process steps do not lend themselves to an efficient overall tinting process.
In contrast to the above, the art also discloses processes for non-covalent incorporation of insoluble vat dyes into hydrogel materials. For example, European Patent No. 0 122 771 B discloses a process wherein the hydrogel material is simultaneously contacted with a swelling agent having a swelling power greater than water and a soluble leuco sulfate ester precursor of the insoluble vat dye in order to permit incorporation of the soluble leuco sulfate ester into the hydrogel material. Subsequently the hydrogel material is treated with an oxidizing agent to convert the soluble leuco sulfate ester to an insoluble vat dye wherein substantially all of the vat dye is physically locked within the hydrogel material. As is apparent this tinting process involves several separate process steps after formation of the hydrogel material.
Other methods for the non-covalent incorporation of a colorant into a hydrogel material includes a first step of incorporating a colorant onto the interior mould surface followed by addition of monomer to the mold and subsequently polymerizing the monomer as described by Rawlings, U.S. Pat. No. 5,034,166. However, this process requires exact placement of the colorant onto the interior mold surface which enhances the difficulty of the process.
Other methods for non-covalent incorporation of tint to hydrogel materials involve including a monomer soluble dye that is water insoluble so as to become trapped in the hydrated hydrogel material. However, such hydrogel materials can be subject to undesirable leaching of the monomer soluble dye which would result in color fading as well as could pose toxicity concerns in patients where the monomer soluble dye could leach during use of a hydrogel material such as use as ophthalmic devices.
Still other known methods for non-covalent incorporation of tint to hydrogel materials (e.g., contact lenses) involve the use of fine dispersions/suspensions of monomer insoluble dyes into the prepolymer mix. Upon polymerization, the insoluble dye is dispersed throughout the hydrogel material. However, one problem associated with this approach is that the monomer insoluble dyes are subject to undesirable aggregate and agglomerate formation.
Accordingly, prior art processes for the non-covalent incorporation of dyes and other colorants into a hydrogel material do not lend themselves to an efficient overall tinting process.
In view of the above, there is a need in the art for a simple method of manufacturing hydrogel materials (e.g., contact lenses) with a tint which does not require extra steps or extra chemicals other than the tinting chemical.